1. Field of the invention
The present invention relates to circuits for synthesizing speech electronically, and in particular to such circuits wherein speech elements are represented by significant components and individual speech elements can be combined into longer speech segments.
2. Description of the Prior Art
Conventional methods for synthetically generating speech elements which may be combined for forming longer speech segments can be generally classified in two groups. The first group includes methods wherein the speech elements are subjected to sampling, the sampling results are converted into digital signals, and are stored in a read only memory from where the sampling results are retrieved (and possibly combined) for speech synthesis. In methods of this type, redundant components of the speech elements which are not necessary for comprehension of the speech elements are also stored, in order to generate a high quality speech reproduction. This requires, however, a correspondingly high storage capacity for representation of such an extensive vocabulary.
The second group of speech synthesizing methods employs substantially the same steps as the methods in the first group, however, redundant speech components are largely supressed and the speech is stored in the form of only significant parameters for each speech element. The speech elements or segments subsequently generated by methods in the second group can nonetheless be comprehended by a listener and moreover can be generated with the need for a significantly lower storage capacity than devices operating according to the first method.
The core of conventional circuits for executing speech synthesis methods in the second group is a filter circuit having variable filter coefficients. Such a speech synthesis circuit is described, for example, in German AS No. 2209548 wherein an excitation signal including significant speech parameters is supplied to a filter circuit having variable filter coefficients. These filter coefficients are continuously controlled by means of further significant speech parameters during the entire synthesis operation, so that this circuit must exhibit devices for precisely storing these filter coefficients. Moreover, this conventional circuit must be equipped with control devices for retrieving the coefficients from the memory and for supplying the coefficients to the filters. Such tunable filters thus require relatively large dimensions and can be realized only with significant circuit outlay and close attention to narrow tolerances required for good speech quality.